June 13, 2022
NASA engineers plan to replace the inclinometer with the inertial measurement unit.
We have been following with excitement as Ingenuity, the small helicopter that accompanied NASA’s Perseverance rover to Mars, landed in the Red Planet’s Jezero Crater in February 2021 and proceeded to make many word first records. Ingenuity has, so far, far exceeded design expectations, adding 28 flights under its belt and beaming back to Earth crucial data for the exploration of the Red Planet.
A dead sensor: the inclinometer
Now, however, the chopper has come across an issue: it has a dead sensor, according to a NASA blog published on June 6.
“Over the past several sols on Mars, the Ingenuity team has been busy recommissioning the helicopter for flight, going through a series of activities that include preflight checkout of sensors and actuators and a high-speed spin of the rotor. These activities have revealed that one of the helicopter’s navigation sensors, called the inclinometer, has stopped functioning. A nonworking navigation sensor sounds like a big deal – and it is – but it’s not necessarily an end to our flying at Mars,” wrote Håvard Grip, Ingenuity Mars Helicopter Chief Pilot at NASA’s Jet Propulsion Laboratory.
What does the inclinometer do?
The inclinometer is built with two accelerometers, whose only purpose is to measure gravity prior to spin-up and takeoff; the direction of the sensed gravity is used to determine how Ingenuity is oriented relative to the downward direction. The inclinometer is not used during the flight itself, but without it, NASA is forced to find a new way to initialize the navigation algorithms prior to takeoff.
Luckily, the tasks assigned to the inclinometer can be taken on by other sensors like the inertial measurement unit (IMU).
“Ingenuity’s sensor suite provides some redundancy when it comes to sensing attitude on the ground. The IMU contains accelerometers, which – just like the accelerometers within the inclinometer – can be used to estimate the initial attitude. Unlike the inclinometer, the IMU is not purpose-built for sensing static orientation, so its initial attitude estimates will generally be somewhat less accurate. However, we believe an IMU-based initial attitude estimate will allow us to take off safely and thus provides an acceptable fallback that will allow Ingenuity to resume flying,” explained Grip in his blog.
Implementing a patch
To proceed with this replacement of tasks, the engineers at NASA must implement a patch, inserting a small code snippet into the software running on Ingenuity’s flight computer, intercepting incoming garbage packets from the inclinometer, and injecting replacement packets constructed from IMU data.
What’s even better is that NASA’s engineers had anticipated that such a situation could arise and had prepared the required software patch prior to last year’s arrival on Mars and kept it on the shelf for this eventuality. One has to marvel at the incredible engineering and preparation of the NASA staff.
